1. Field of the Invention
The present invention relates to a communication system, a transmitter and a receiver, and is applicable, for example, to a mobile radio communication system, such as, a personal communication system or a digital cellular system, and a mobile station and a base station employed therein, and is further applicable to a transmitter-receiver of a mobile radio switching system.
2. Description of the Prior Art
Many conventional mobile radio communication systems follow a standard (literature 1) entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95" published as an international standard of the Telecommunications Industry Association/Electronics Industry Association and a standard (literature 2) entitled "W-CDMA (Wideband Code Division Multiple Access) Air Interface Compatibility Standard for 1.85 to 1.99 GHz PCS Applications, IS-665" proposed at the Joint Technical Committee thereof.
The literature 1 describes a technique for time-division allocating connection control data into frames of voice data, which is user data, for multiplex transmission on a traffic channel either from a base station to a mobile station or from a mobile station to a base station.
FIG. 4 shows a down-link structure from a base station to a mobile station according to the literature 1, and FIG. 5 shows an up-link structure from a mobile station to a base station according to the literature 1.
In the down-link structure shown in FIG. 4, the base station BS and the mobile station MS are connected to each other via a radio propagation path 107. The base station BS comprises a pilot signal spreading circuit 101 to be inputted with a pilot signal "all 0's", a time-division multiplex (TDM) frame generator 102 for time-division multiplexing connection control data and user data, a user signal spreading circuit 103, an adder 104, a carrier modulator 105 and an antenna 106. On the other hand, the mobile station MS comprises an antenna 108, a carrier demodulator 109, a user signal despreading circuit 110, a pilot signal despread circuit 112, a multiplier 111, a conjugate conversion circuit 113 and a TDM frame demultiplexer 114.
In the up-link structure shown in FIG. 5, the mobile station MS and the base station BS are connected to each other via a radio propagation path 206. The mobile station MS comprises a TDM frame generator 201 to be inputted with connection control data and user data, a Walsh spreading circuit 202, a user signal spreading circuit 203, a carrier modulator 204 and an antenna 205. On the other hand, the base station comprises an antenna 207, a carrier demodulator 208, a user signal despreading circuit 209, a Walsh despreading circuit 210 and a TDM frame demultiplexer 211.
With the foregoing arrangement, the TDM frame generator 102 of the base station BS in the down link and the TDM frame generator 201 of the mobile station MS in the up link suitably multiplex user data, such as voice data, and connection control data in each of traffic channel frames having a data region of 168 bits. The data allocation is determined by the transmission rate of the user data and classified into five frame formats, that is, transmission of only the user data, transmission of 1/2 user data and 1/2 connection control data, transmission of 1/4 user data and 3/4 connection control data, transmission of 1/8 user data and 7/8 connection control data and transmission of only the connection control data. In case of the 9600bps transmission rate, the ratios between the number of user data bits and the number of connection control data bits per frame are 171/0, 80/88, 40/128, 16/152 and 0/168 in the foregoing five frame formats, respectively. Due to difference in error correcting methods, the ratios of 80/88, 40/128 and 16/152slightly deviate from the ratios of 1/2:1/2, 1/4:3/4 and 1/8:7/8, respectively. Using the foregoing frame formats, the ratio between the user data and the connection control data is adjusted according to the amount of the user data.
In the down link shown in FIG. 4, the multiplexed data from the TDM frame generator 102 is spread at the user signal spreading circuit 103 and then given to the adder 104. On the other hand, a common pilot channel including no data is spread at the pilot signal spreading circuit 101 and also given to the adder 104. At the adder 104, the spread signals from the circuits 101 and 103 are multiplexed. Then, at the carrier modulator 105, modulation, for example, phase shift keying (PSK), is carried out using the multiplexed spread signal from the adder 104 as a modulating signal to obtain a PSK modulated carrier. The PSK modulated carrier is transmitted to the mobile station MS from the antenna 106. The PSK modulated carrier is received at the antenna 108 of the mobile station MS via the radio propagation path 107 and then demodulated at the carrier demodulator 109 to obtain the modulating signal. The modulating signal is then fed to the user signal despreading circuit 110 and the pilot signal despreading circuit 112 so as to be despread respectively. The pilot signal, in the form of a complex number signal given from the pilot signal despreading circuit 112, is subjected to a conjugate conversion at the conjugate conversion circuit 113 and is then given to the multiplier 111. At the multiplier 111, an output signal from the user signal despreading circuit 110 is multiplied by the pilot signal from the conjugate conversion circuit 113 so that a random phase rotation of the output signal from the circuit 110 caused by fading in the radio propagation path is corrected. Then, the corrected output signal is demultiplexed into the connection control data and the user data at the TDM frame demultiplexer 114.
On the other hand, in the up link shown in FIG. 5, the multiplex data from the TDM frame generator 201 is Walsh spread by the M-ary spreading at the Walsh spreading circuit 202. Then, the multiplexed data is spread at the user signal spreading circuit 203 and given to the carrier modulator 204. At the carrier modulator 204, modulation, for example, phase shift keying (PSK), is carried out using the multiplexed spread signal from the circuit 203 as a modulating signal to obtain a PSK modulated carrier. The PSK modulated carrier is transmitted to the base station BS from the antenna 205. The PSK modulated carrier is received at the antenna 207 of the base station BS via the radio propagation path 206 and then demodulated at the carrier demodulator 208 to obtain the modulating signal. The modulating signal is then fed to the user signal despreading circuit 209, where it is despread Thereafter, the despread signal from the circuit 209 is given to the Walsh despread circuit 210, where it is Walsh despread The despreading signal from the circuit 210 is then demultiplexed into the connection control data and the user data at the TDM frame demultiplexer 211.
In the up link, as described above, the spreading and the despreading are carried out with Walsh functions so as to establish asynchronous communication where no pilot signal is used. In this asynchronous communication, an information bit sequence is spread into orthogonal code sequences at the transmitter side, and the sequence having the greatest correlation value is selected at the receiver side.
FIG. 6 shows an up-link structure from a mobile station to a base station according to the literature 2.
FIG. 6, the mobile station MS comprises a pilot signal spreading circuit 301, a connection control signal spreading circuit 302, a user signal spreading circuit 303, an adder 304, a carrier modulator 305 and an antenna 306.
In the mobile station MS, a pilot signal, connection control data and user data are spread at the spreading circuits 301, 302 and 303, respectively, and then superposed at the adder 304. The user data (voice data or the like) is spread into in-phase (I-phase) components and quadrature-phase (Q-phase) components using a quadrature pair of different spreading codes. Thereafter, at the carrier modulator 305, modulation, for example, phase shift keying (PSK), is carried out using an output signal from the adder 304 as a modulating signal to obtain a PSK modulated carrier. The PSK modulated carrier is transmitted to the base station BS from the antenna 306. The PSK modulated carrier is received at an antenna 308 of the base station BS via a radio propagation path 307 and then demodulated at a carrier demodulator 309 to obtain the modulating signal. The modulating signal is then fed to a user signal despreading circuit 310, a connection control signal despreading circuit 312 and a pilot signal despreading circuit 314 so as to be despread respectively. The pilot signal in the form of a complex number signal given from the pilot signal despreading circuit 314 is subjected to a conjugate conversion at a conjugate conversion circuit 315 and is then given to a multiplier 311 and a multiplier 313. At the multiplier 311, an output signal from the user signal despreading circuit 310 is multiplied by the pilot signal from the conjugate conversion circuit 315 so that the user data is obtained. Similarly, at the multiplier 313, an output signal from the connection control signal despreading circuit 312 is multiplied by the pilot signal from the conjugate conversion circuit 315 so that the connection control data is obtained. As in the foregoing down link according to the literature 1, the pilot signal is used for correcting a random phase rotation of the signals caused by fading in the radio propagation path.
In the mobile radio communication system according to the literature 1, since the user data and the connection control data are time-division multiplexed on the traffic channel, the transmission rate of the user data is lowered.
Further, time-division multiplexers are required for all the traffic channels, and controllers are required for determining allocation of the user data and the connection control data into the traffic channel frames. Accordingly, the hardware becomes complicated in structure and enlarged in size.
The framing operation for the user data and the connection control data further causes a transmission delay of the user data. Further, voice detection circuits are necessary, which further makes it difficult to design the whole communication system.
Further, in the asynchronous communication from the mobile station to the base station, since the pilot signal is not used, errors in symbols as a result of the spreading of the information are increased and, for correcting them, the redundant M-ary spreading is required. This lowers the transmission rate.
In the mobile radio communication system according to the literature 2, the foregoing problems are eliminated to a large extent. However, since the pilot channel and the connection control channel are multiplexed on the I-phase and Q-phase information channels, interference occurs among those channels and further occurs relative to other mobile stations communicating with same base station.